This invention relates generally to a bone reinforcement process and surgical tool for, and more particularly, the present invention relates to an application device for injecting poly methyl methacrylate into a bone matrix through a canulated element through which a screw may subsequently be inserted.
The bones and connective tissue of an adult human spinal column consists of an upper portion having more than 20 discrete bones, and a lower portion which consists of the sacral bone and the coccygeal bodies. The bones of the upper portion are generally similar in shape, however, they do vary substantially in size in accordance with their individual position along the column and are, therefore, anatomically categorized as being members of one of three classifications: cervical, thoracic, or lumbar.
These similarly shaped bones vary in size, but are each similarly coupled to the next by a tri-joint complex. The trijoint complex consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. The posterior portion of the vertebral bone is coupled to the anterior portion by a pair of bone bridges referred to as pedicles, between which the spinal canal is housed.
In its entirety, the spinal column is highly complex in that it houses and protects critical elements of the nervous system which have innumerable peripheral nerves and arterial and veinous bodies in close proximity. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist through a wide range of motion.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classification suggests, posterior implants are attached to the back of the spinal column, generally hooking under the lamina and entering into the central canal, attaching to the transverse process, or coupling through the pedicle bone. Lateral and anterior assemblies are coupled to the vertebral bodies.
The region of the back which needs to be immobilized, as well as the individual patient""s anatomy, determine the appropriate surgical protocol and implantation assembly. Because the spine is routinely subject to high loads which cycle during movement, primary concerns of physicians performing spinal implantation surgeries focus on screw pull-out and screw failure. Screw pull-out occurs when the cylindrical portion of the bone which surrounds the inserted screw fails. Screw pull-out often an additional danger in that it often leaves the bone into which the screw was implanted completely useless with respect to continued implant support. This is especially true when the patient suffers from osteoporosis. In such patients the bone matter is often much less structurally supportive and lacks the necessary holding strength to prevent macromotion of the screws which may be implanted therein, thus severely limiting the immobilization potential of the assembly.
The use of artificial materials, such as bone cements and specific organic bone mimicking compounds such as poly methy methacrylate (PMMA), have been taught in the art as being effective in strengthening the osteoporotic bones to effect better immobilization of the screws. Percutaneous insertion of bone reinforcing agents has been successful in many instances, and is generally known as vertebroplasty. This xe2x80x9cclosedxe2x80x9d use of PMMA and/or bone cement is useful in supporting subsiding bone masses in some instances, but is insufficient in those cases in which pedicle screw support is required. One of the failings of vertebralplasty, however, is that the cured PMMA/bone cement is often so much more dense and hard than the surrounding natural bone material that if subsequent screws need to be inserted, the bone drill is confounded by the difference in material properties.
The xe2x80x9copenxe2x80x9d use of PMMA and/or bone cement has been thought of as an alternative to xe2x80x9cclosedxe2x80x9d use, especially when posterior implants are expected to be utilized. In such an instance, the patient""s posterior spine is exposed and a bone drill is used to bore a hole through the pedicles for the posterior assembly to be implanted. Prior to the screws being implanted, however, the surgeon injects a quantity of PMMA/bone cement into the hole. Subsequently, the screw is inserted into the hole with the uncured cement. As the cement harden around the threads of the screw, however, the screw becomes thoroughly incarcerated in the hole, and is thus irretrievable. This presents a significant problem for potential revision surgery as well as being a cumbersome and time sensative process (as the PMMA/bone cement must not dry before the screw is implanted.
It is, therefore, the principal object of the present invention to provide a bone cement injector system for use in spine surgery wherein the surgeon has the ability to assemble the bone cement injectors without the time pressure of inserting the screws exactly after the material has been inserted.
It is also an object of the present invention to provide a bone cement injector system for use in spine surgery wherein the surgeon has the ability to insert the pedicle screws into a dried bone cement cavity which will support, but not incarcerate the screw against removal if necessary.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.
The preceding objects are achieved by the present invention, which is a system and method for reinforcing bone in preparation for screw implantation. A system of the invention in one embodiment comprises a threaded cannula having a central bore and a perforated distal end, a cannula applicator that is insertable into the central bore and which achieves a friction fit within the central bore, a plunger that is insertable into the central bore and which achieves an intimate fit within the central bore (the plunger having a guide wire passing through its central longitudinal axis), bone cement, and a cannulated drill bit. A method of the invention in one embodiment comprises drilling and tapping a hole in a vertebral body, inserting the applicator into the central bore of the cannula, screwing the cannula into the tapped hole by rotating the applicator, removing the applicator, injecting the bone cement into the central bore, distributing the bone cement out the holes in the distal end of the cannula and into the surrounding bone using the plunger, letting the bone cement harden, and drilling out the plunger using the cannulated drill following the guide wire. Thereafter, the surgeon can re-tap the hole and insert a bone screw into the reinforced vertebral body.
More particularly, a cannula of the invention has an elongated cylindrical body with a central bore, the body having a proximal end providing access to the bore (especially access by a cannula applicator, plunger, syringe and drill bit of the present invention, as described in greater detail below), and a distal end that is perforated. The outer surface of the cannula is threaded for engagement with threads of a tapped drill hole and to restrict proximal migration of the bone cement, as described in greater detail below. The cannula should be formed from biocompatible material (e.g., poly methyl methacrylate) inasmuch as it will become incarcerated into the target vertebral body in accordance with the procedures described herein. Preferably, the cannula has a radiodense tip that can be used to aid the surgeon in determining the position of the cannula after the cannula has been placed into the target vertebral body.
A cannula applicator of the invention has an elongated cylindrical body and is used to assist the surgeon in threading the cannula into a tapped drill hole and in determining the placement of the cannula in the vertebral body, as described in greater detail below. Accordingly, the applicator is dimensioned so that it can be placed into and removed from the proximal end of the cannula and so that when the applicator is placed into the bore of the cannula, it fits snugly within the bore. The intimate fit enables the applicator to provide structural support for the cannula as the cannula is twisted into the drill hole, and causes the applicator to grip the walls of the bore so that cannula will rotate when the applicator is rotated, so that the cannula will threaded into the drill hole. Preferably, the applicator comprises a radiodense material or is of a radiodense configuration, so that it can be used to determine the position of the cannula as the cannula is threaded into the drill hole.
A plunger of the present invention has an elongated cylindrical body that fits tightly within the bore of the cannula so that it can be used to squeeze bone cement out the holes in the distal end of the cannula as described in greater detail below. Preferably, the body is formed from a material that is softer than the biocompatible material from which the body of the cannula is formed. As described in greater detail below, this difference in material facilitates the drilling away of the plunger after it is used to distribute the bone cement. Also preferably, the plunger is formed from biocompatible material (e.g., poly methyl methacrylate), as some of the plunger may remain after most of the plunger has been drilled away, and the remaining portion would become incarcerated in the vertebral body. Also preferably, the body has a central longitudinal axis and an internal guide wire passing through the central longitudinal axis. As described in greater detail below, this guide wire also facilitates the drilling away of the plunger.
During use of the invention, upon proper preparation of the target vertebral body or bodies in accordance with known and accepted surgical procedures, the surgeon drills a hole in the target vertebral body. Then, the surgeon threads the hole using a tap in a manner known in the art. The surgeon repeats the above procedure for each hole he wishes to drill.
Next, the surgeon inserts into the hole a cannula of the present invention, using an appropriately sized cannula applicator of the present invention. The surgeon inserts the applicator into the proximal end of the cannula and into the bore of the cannula, establishing a tight fit of the applicator against the walls of the bore. Once the applicator is fitted into the bore, the surgeon places the cannula into the tapped hole, and repeatedly turns the applicator to screw the cannula into the hole to the desired position (typically, all the way into the hole). The intimate fit of the applicator in the bore facilitates the rotation of the cannula in response to the rotation of the applicator. The structural integrity of the applicator, in conjunction with the intimate fit of the applicator in the bore, provides structural support for the thin-walled cannula as the cannula is twisted into position. For each drilled hole, the surgeon places a cannula into the drilled hole using an appropriately sized applicator in accordance with the above procedure. The surgeon should leave each applicator in place until it is time to inject the bone cement, as described below. This will keep bleeding to a minimum and will continue to make possible radiographic assessments of the position of each applicator and accordingly each cannula.
Once each drilled hole has been fitted with a cannula, the surgeon prepares the appropriate bone cement mixture and loads one or more syringes with the bone cement, in a manner know in the art. Then, for each installed cannula, one at a time, the surgeon removes the applicator, injects an appropriate amount of the bone cement into the bore using the syringe(s), and applies a plunger of the present invention to distribute the bone cement through the holes of the distal end of the cannula. In order to effect this procedure for each cannula, the surgeon first removes the applicator from the cannula by pulling it from the bore. Next, the surgeon prepares the bone cement, loads the syringe(s), and injects the bone cement into the bore. Then, the surgeon inserts an appropriately sized plunger of the present invention into the distal end of the cannula and into the cannula bore, pushing the plunger down the bore so that the bone cement squeezes out the holes at the distal end of the cannula and into the bone surrounding the cannula. For each cannula, the surgeon leaves the plunger in until each plunger has been applied and the bone cement has set in the surrounding bone. The setting of the bone cement in the surrounding bone strengthens the surrounding bone in preparation for the next steps, which involve re-tapping the target vertebral body for a bone screw.
Once each plunger has been applied and the distributed bone cement has set, the surgeon drills out each plunger using a drill and cannulated drill bit. The surgeon selects a cannulated drill bit having an appropriate outer diameter, sets the drill bit into the drill, passes the drill bit over the guide wire extending from the plunger, and proceeds to drill into the plunger body, following the guide wire to ensure that primarily the plunger body is being drilled away. As noted above, the preferable softness of the plunger body relative to the cannula body facilitates the drilling away of primarily the plunger body. The surgeon repeats this procedure for each installed cannula.
Finally, the surgeon threads each hole that remains after each plunger has been removed, using a tap in a manner known in the art. Once each new hole has been tapped, the surgeon can insert a bone screw of the surgeon""s choice into each hole, and complete the operation.